This invention relates to a ratio discriminator.
As is known, discriminators are generally passive quadripoles which can provide at the outputs a voltage proportional to the deviation of an input signal from a set center frequency. Such discriminators are widely employed in frequency modulation radio receivers to extract the information from frequency modulated signal, as well as in amplitude modulation radio receivers for recognizing the carrier following conversion.
In particular, widely utilized as discriminators are the so-called ratio discriminators, which are characterized by the output voltage being largely independent of any amplitude variations in the input signal. This feature is advantageous in that the ratio discriminator enables those limiters which are required by other discriminator types to suppress amplitude variations, to be eliminated. A typical ratio discriminator is shown, for example, in FIG. 1. That discriminator broadly comprises an intermediate frequency modulated signal input, represented by the transistor 1, a first oscillating circuit 2 and a second oscillating circuit 3, inductively coupled to each other, and a detector circuit 4 having an output electrode 5. In detail, the first oscillating circuit comprises a capacitor 6 and primary winding 7 having an intermediate tap 8 connected to the collector of the transistor 1, while the second oscillating circuit comprises a capacitor 9 and secondary winding 10, also having an intermediate tap 11 connected to a tertiary winding 12 which is grounded at the other of its terminals. The detector is also provided with a pair of diodes 12 and 13 which are connected, the one with the anode and the other with the cathode, to the second oscillating circuit 3, as well as resistors 14,15,16 and 17, and capacitors 18,19 and 20. That circuit is well known and sold, for instance, by Toko and Sumida, and will not be detailedly discussed herein below. It will suffice to say that that circuit has an output characteristic of the type shown in FIG. 2, wherein the output voltage for a given frequency range behaves approximately linearly with a zero crossing where the frequency of the modulated input signal is exactly at the center of the band of the discriminator. In actual applications, there is a need for exactly detecting that zero crossing in order to assess whether a receiver incorporating said discriminator is tuned on a desired frequency, and possibly assist in channel seeking. However, the detection of that zero crossing has posed some problems heretofore owing to the difficulty of detecting the zero crossing in arrangements having a single power supply with the radio receivers, and to the lack of a reliable DC signal (as due to thermal drifts, changes in the components or in the power supply, etc.) in discriminator receivers which inherently introduce a DC signal component.